Marek's Disease (MD) is a malignant, lymphoproliferative disorder of domestic fowl caused by a herpesvirus: Marek's Disease Virus (MDV). MD is ubiquitous, occurring in poultry-producing countries throughout the world. Chickens raised under intensive production systems will inevitably suffer losses from MD. MD affects chickens from about 6 weeks of age, occurring most frequently between ages of 12 and 24 weeks.
Three forms of MD are recognized clinically, classical MD, acute MD and transient paralysis.
Classical MD is characterized by peripheral nerve enlargement caused by lymphoid infiltration and demyelination, and paralysis is the dominant clinical sign. Mortality is variable but normally under 10-15 percent.
In the acute form there are multiple and diffuse lymphomatous tumours in the visceral organs. Mortality from this form of MD is usually higher than from the classical form. An incidence of 10-30 percent is common in unvaccinated flocks and outbreaks involving up to 70% of the flock may be encountered. The pathological lesions in both classical and acute MD are essentially similar, involving the proliferation and infiltration of malignantly transformed T-lymphoblasts into normal tissues, peripheral nerves in the case of the classical form and visceral organs in the case of the acute form.
Furthermore, the MDV has been shown to be responsible for encephalitis of young chickens characterized by sudden paralysis.
Serological classification of MD related viruses yielded three serotypes:
Type I : naturally occurring virulent strains of Marek's disease-virus which are pathogenic and tumorigenic to chickens, and attenuated nonpathogenic strains derived there from PA1 Type II: naturally occurring nonpathogenic strains of Marek's disease virus; and PA1 Type III: herpesvirus of turkeys ("HVT"), which is nonpathogenic to chickens.
There are several practical Marek's disease vaccine types currently in use. These include vaccines derived from pathogenic serotype 1 strains of MD virus. Serial passage of these strains was found to result in loss of pathogenicity and oncogenicity, but not of immunogenicity. Attenuated viruses derived from strain HPRS-16, the prototype MD vaccine (Churchill, A. E. et al., J. Gen. Virol. 4, 557, 1969) and the CVI-988 strain have already been licensed for commercial use as a live serotype 1 MD vaccine.
Serotype 2 MD viruses are naturally non-oncogenic and thus do not have the potential for causing tumours in vaccinated chickens. Therefore, these viruses do not require any artificial attenuation by serial passaging and since they are in their natural state, can not revert to a virulent form. The HN-1 strain has been shown to be successful in vaccination in addition to the SB-1 strain (U.S. Pat. No. 4,160,024) which has been licensed in the United States since 1983. Hitherto, serotype 1 and serotype 2 vaccines have to be administered as cell-associated preparations (Powell, P. C., World's Poultry Science Journal 42, 205, 1986; Witter, R. L. et al , Avian Diseases 31, 829, 1987; Schat, K. A., Internews 3, 13, 1989). In practise, this means that storage and transportation of said vaccines have to take place in liquid nitrogen at about -196.degree. C.
Errors in vaccine storage and handling result in the decrease of the viability of the MD viruses and cause failure of the vaccination. In particular in countries where liquid nitrogen storage is practically impossible cell-associated MD vaccines are not applicable. Furthermore, the MDV containing particles suspended in a cell-associated vaccine precipitate, require the homogenization of the suspension before administration. Inadequate homogenization may result in an incorrect dose of vaccine and therefore in a failure of the vaccination. Moreover, the strictly cell-associated nature of said vaccines is responsible for the susceptibility of the vaccines to physical abuse. Damage to the infected cells by sub-optimal harvesting and freezing procedures as well as incorrect thawing of the ampules and handling of the vaccine at the hatcheries will cause cell damage and death and subsequent loss of vaccine titres.
Nowadays, a frequently used MD vaccine is derived from HVT. HVT was first isolated from turkeys, is apathogenic in turkeys and in fowls and is antigenically related to serotype 1 and 2 MD viruses. HVT is extensively used as a vaccine against MD, the FC126 strain being most widely used. HVT is commonly used as a cell-associated preparation, but because substantial amounts of cell-free virus can be extracted from infected cells, it may also be used as a lyophilized, cell-free vaccine.
Because of the continued pressure to reduce economic losses from MD to lower levels a need exists to continuously improve the efficacy of MD vaccines, especially now that excessive losses in the poultry industry as a result of the occurrence of very virulent strains of MD virus have been reported both in the US and in Europe. Thusfar, HVT vaccination does not offer adequate protection against such isolates, even at high doses or after extended intervals between vaccination and challenge. The spread of these very virulent field strains of MD virus will be favoured by the relatively inefficient vaccination with HVT alone.
A useful method currently available to control disease caused by infection with the very virulent MD viruses is the use of bivalent or polyvalent vaccines containing mixtures of vaccine viruses belonging to the different serotypes of the MD virus group. It was found that a bivalent vaccine composed of HVT and SB-1 or another serotype 2 MD virus provided better protection than any component virus alone. This phenomenon was termed "protective synergy" designating the mechanism by which the magnitude of the protection afforded by one MD vaccine virus is augmented by the addition of a second vaccine virus (Witter, R. L., Avian Pathology 11, 49, 1982; Witter, R. L. and Lee, L. F., Avian Pathology 13, 75, 1984; Witter, R. L., Avian Diseases 31, 752, 1987; Schat, K. A. et al., Avian Pathology 11, 593, 1982). Disadvantageously, in order to benefit from this synergy the bivalent vaccine has to be a cell-associated preparation as until now a cell-free serotype 2 MD virus vaccine is not available.
MDA (maternal derived antibodies) to all MD viral serotypes are ubiquitous in commercial chicks due to natural exposure of breeders to MD viruses and/or vaccination of breeders with serotype 1, 2 and 3 viruses. The adverse effect of homologous MDA on vaccination is generally known. MDV antibodies do interfere with (cell-free) HVT vaccine only at a low level. Thus, it is advantageous to be able to vaccinate breeder flocks with HVT-lacking MD virus vaccines in order that their progency might be better protected with HVT. This so-called alternate generation vaccination has potential merit even when the progeny are vaccinated with HVT-containing bivalent or polyvalent vaccines. However, application of this vaccination strategy requires the availability of a satisfactory HVT-lacking vaccine (Witter, R. L. and Lee, L. F. Avian Pathology 13, 75, 1984). The availability of a vaccine containing a cell-free serotype 2 MD virus, e.g. SB-1, would be very useful in this alternate generation vaccination.
According to the present invention a vaccine is provided for the protection of poultry against MD, characterized in that this vaccine comprises cell-free MD serotype 2 viruses together with a pharmaceutically acceptable carrier.